Macintosh Classic II: Low Voltages at Start Up

[sadmanonatrain]

Hello all,

In September last year I bought a faulty Macintosh Classic II (40Mb HD, 4Mb RAM) with the intent of restoring it.

To restore it to its former glory I recapped the motherboard and treated its case with Retrobright.

I reassembled the Macintosh and turned it on to be confronted with a screen raster that appears to 'wobble'. This will happen for about 15-20 minutes before the voltages rise and stabilise, which makes the screen raster stationary.

I thought that the power supply portion of the (International 240v) analog board was going bad so I replaced pretty much all of the capacitors I could find, even the large 220uf 400WV one!

But alas, replacing capacitors did not help; the raster was still 'wobbled' on startup.

To investigate further I measured voltages from the floppy port with my multi-meter. For example: the 5v starts at 4.80 then slowly rises to about 5.29 where the raster stabalises. At about 5.09 the HD starts to spin up.

The Macintosh needs to warm up before operating normally.

So here I am, one year later, with a Classic II that still exhibits the same voltage problems.

I don't know what to do next!

Any help would be much appreciated!

 

[uniserver]

Did you replace these caps? - they are the only ones that really need to be changed anyways.

these.

viewtopic.php?f=7&t=21471&start=0

also can you take a high res image with flash of the mainboard.?

to have a look and see if there is any dark traces etc.

 

[sadmanonatrain]

uniserver,

Did you replace these caps?

I have replaced all of the eight caps you recommended the replacement of. Unfortunately this did not solve the problem!

I've uploaded some analog and logic board photographs:











I've noticed on CP38, a RIFA component, that its case has a crack on its top.

I hope this helps!

 

[uniserver]

2 things.

looks like there is a cap you missed.

and the leads are corroded so its bad.



I would also get some acetone or nail polish remover… and a tooth brush

and try to get that burned up flux cleaned up so you can better see what is going on.

I know the classic II has all kinds of traces they run in-between the cap pads. witch makes it really easy to bridge them.

Charles

 

[uniserver]

another thing you can do is…

Give all those pots like a 1mm of turn then run them back to their original spot, in the rear where all the pots are lined up.

I noticed sometimes those pots need some movement for the values to return to normal.

 

[techknight]

Get a can of compressed air, and turn it upside down.

Start hitting components 1 at a time. After its warmed up and running.

Start with the 6 pin opto-isolator. you WILL find your fault this way.

Also did you replace the little electrolytics around the TDA IC? you cannot leave the little guys out on the analog board either. its not always the big ones. its ALL of them. Multiple years of electronics experience has taught me one thing: the little small electrolytics leak far before the large ones do, unless the large ones are run much harder. The symptoms are signs of poor regulation in the feedback network.

 

[sadmanonatrain]

Hello all,

uniserver: Thanks for spotting that capacitor! It has now been replaced.

techknight: I didn't have any cans of compressed air so I gave the analog board a good scrub down with some rubbing alcohol and a toothbrush. I focused on the two components you mentioned.

I reassembled the the Macintosh and left it overnight. The next morning I switched it on and now it is not exhibiting any of the 'wobbly' raster behavior!

I'll do some more testing but I think that's the problem fixed!

Thanks for all the help

 

[uniserver]

- great man!